In USB (Universal-Serial Bus) or IEEE 1394 Bus, every device will be given one and only  serial number or ID. Generally speaking, the processes for generating a serial number at present can be divided into two kinds. Wherein one kind  One of the processes for generating a serial number is to use a fixed serial number on devices of the same kind. Although this process is simple and convenient, the devices of the same kind cannot be connected to one and the same USB or IEEE 1394 Bus and be used normally.
The other commonly known process for generating a serial number is to store the serial number of devices in ROM, EPROM, EEPROM or flash memory, in order to make the serial number one and only . However, this process will cause the increase of the cost for programming the serial number and the  increase of  the expenses for memory circuit components in the process of manufacturing the devices. Apart from this, it is necessary to have control to prevent the serial number from repetition in the process of manufacturing, and in order to have the function of control, a related control circuit must be provided accordingly.